1. Field of the Invention
The present teachings relate to a method of preparing electrocatalyst compositions containing platinum and tungsten by thermally decomposing carbonyl complexes of the two metals.
2. Discussion of the Related Art
The desire to reduce the amount of expensive platinum group metals needed to obtain a desired level of performance for an electrocatalyst is an ever present operational parameter for the design of many devices and procedures involving electrocatalysts.
Enhancement of catalytic performance by combining platinum with various less expensive metals is one of the possible avenues to reduce the amount of platinum required, or increase the efficiency of the catalyzed reaction, or both. One application of interest for Pt-based electrocatalyst is, for instance, the cathode reaction of a typical proton exchange membrane fuel cell (“PEMFC”) which can utilize an electrode containing a Pt-based electrocatalyst to catalyze the oxygen reduction reaction (“ORR”).
Proton exchange membrane fuel cells are promising power sources for transportation, portable and stationary applications. The sluggish kinetics of ORR at the cathode and the high cost of platinum electrocatalysts are just some of major challenges that impede the commercialization of PEMFCs. Various studies have found that platinum alloys to have higher electrocatalytic activities compared to the standard platinum on carbon formulation. See, for instance B. C. Beard, P. N. Ross, J. Electrochem. Soc. 130 (1990) p. 3368; S. Muketjee, S. Srinivasan, J. Electroanal. Chem. 357 (1993) p. 201; M. Watanabe, K. Tsurumi, T. Mizukami, T. Nakamura, P. Stonehart, J. Electrochem. Soc. 141 (1994) p. 2659; A. S. Arico, A. K. Shukla, H. Kim, S. Park, M. Min, V. Antonucci, Appl. Surf. Sci. 172 (2001) p. 33; E. Antolini, R. R. Passos, E. A. Ticianelli, Electrochim. Acta 48 (2002) p. 263; U. A. Paulus, A. Wokaun, G. G. Scherer, T. J. Schmidt, V. Stamenkovic, N. M. Markovic, P. N. Ross, Electrochim. Acta 47 (2002) p. 3787; L. Xiong, A. Manthiram, J. Mater. Chem. 14 (2004) p. 1454; L. Xiong, A. Manthiram, J. Electrochem. Soc. 152 (2005) p. A697; E. Kreidler, L. Minor, L. Xiong, T. He, Electrocatalysis (Eds. G. Brisard, R. Adzic, V. Birss and A. Wieckowski, The Electrochemical Society, Pennington, N.J.) PV11 (2005) p. 222; and J. Luo, N. Kariuki, L. Han, L. Wang, C. J. Zhong, T. He, Electrochim. Acta 51 (2006) p. 4821.
However, recent studies have reported on the instability of platinum alloy electrocatalysts under PEMFC operating conditions. See, for instance, T. R. Ralph, M. P. Hogarth, Platinum Met. Rev. 46 (2002) p. 3 and J. Xie, D. L. Wood III, D. M. Wayne, T. A. Zawodzinski, P. Atanassov, R. L. Borup, J. Electrochem. Soc. 152 (2005) p. A104.
The observed instabilities of these alloys has been attributed to the dissolution of 3d transition metals in the platinum alloys, with some cases reported of diffusion into the membrane or even migration to the anode. Therefore, the stability issue of the electrocatalyst formulation has been a major concern for their application as electrodes in PEMFC.
Carbon supported PtW alloys have been reported, but only as the anode electrocatalysts for PEMFC operated on methanol, ethanol, or reformate gases. See, for example, M. Götz, H. Wendt, Electrochim. Acta 43 (1998) p. 3637; W. Zhou, Z. Zhou, S. Song, W. Li, G. Sun, P. Tsiakaras, Q. Xin, Appl. Catal. B 46 (2003) p. 273; A. S. Aricò, A. Poltarzewski, H. Kim, A. Morana, N. Giordano, V. Antonucci, J. Power Sources 55 (1995) p. 159; S.-A. Lee, K.-W. Park, J.-H. Choi, B.-K. Kwon, Y.-E Sung, J. Electrochem. Soc., 149 (2002) p. A1299, and M. Goetz, H. Wendt, J. Appl. Electrochem. 31 (2001) p. 811.
Carbon supported platinum tungsten alloy nanoparticles have been prepared by impregnation or chemical co-reduction of chloroplatinic acid and ammonium tungstates. See the M. Gotz and W. Zhou references listed above. Drawbacks to those processes include the inability to control particle size and compositional homogeneity, and difficulty in reducing tungsten to the metallic state.
A need exists for methods of producing supported platinum tungsten containing electrocatalysts that reduce the amount of platinum needed to achieve a desired performance level.